I don’t entirely disagree with your math. A united Democratic Party could certainly have come a great deal closer and possibly won the election.
However, comparing the 1860 election to the 1856 election is an exercise in futility. 1856 wasn’t a “normal” election. The opposition to the Democrats was split several ways, so it was as big an advantage for the Democrats as the 1860 election was for the Republicans.
In the 1852 election the Whigs were in the process of falling apart, over the issue of slavery, oddly, despite slavery being only a minor issue at the time.
So the last really “normal” two-party election was that of 1848, which was won by the Whigs.
BTW, in 48, 52 and 56 the winner drew a minority of the popular vote, just like in 1860. In fact, as the nation became more polarized over the unimportant issue of slavery, the percentage of the popular vote taken by the winner went down with each of these election.
But I'm not doing anything mathematically sophisticated here, just counting up how many combined votes Northern and Southern Democrats received in each state.
Doing that shows: in addition to the 84 electoral votes Democrats won, a combined ticket would have won another 46 (California, Kentucky, Oregon, Tennessee & Virginia) giving them 130 -- only 22 short of victory.
Indiana and Illinois had 24 votes between them, and only 11,000 voters switching from Republican to Democrat would give Dems the victory, and the presidency.
And, I'm saying those 11,000 represents the "enthusiasm gap" Democrats suffered from their split.
The same was true in Ohio and Pennsylvania.
So the 1860 election didn't even have to be close.